1. Field of the Invention
This invention relates to printing systems and methods and particularly systems and methods using multiple scan beams that have wide lateral separations.
2. Description of Related Art
Printing systems including scanners are suitable for a variety of applications including printing text on paper, patterning photoresist during integrated circuit manufacture, and creating masks or reticles for projection-type photolithography systems. For integrated circuit applications, the printing systems typically require submicron precision. FIG. 1A illustrates the basic architecture of a precision printing systems 100 that employs scanning. System 100 includes: a light source 110 such as a laser; an acousto-optic modulator 120 that controls intensity of one or more input beams 135; pre-scan optics 130 that control the position, shape, and collimation of input beams 135; a scanning element 140 such as a polygon mirror that sweeps scan beams 145 along a scan direction; and post-scan optics 150 that focus scan beams 145 on an image plane 160. Scanning of scan beams 145 forms scan lines that expose a pattern in an image area of plane 160. Acousto-optic modulator 120 modulates the intensity of input beams 135 to select the pattern that scan beams 145 expose.
A conventional acousto-optic modulator includes a block of material such as fused silica through which input beams propagate. To turn on, turn off, or change the intensity of an input beam, a transducer generates an acoustic wave that crosses the path of the input beam in the block. The acoustic wave locally changes the optical properties of the block and deflects part of the input beam. Typically, a beam stop later in the optical train blocks the undeflected portion of the beam.
A concern for a precision scanner having a conventional acousto-optic modulator is the orientation of the scanning direction relative to propagation of the acoustic waves that modulate the input beams. If the propagation direction and the scanning direction are not collinear, the turning on and turning off of beams can reduce sharpness of edges or create undesired skew or directional bias in a pattern being illuminated. FIG. 1B illustrates an illuminated region 170 of a scan line formed when an acoustic wave deflects an input beam in a direction 178 that (after convolution through the system optics 130 and 150) is perpendicular to a scan direction 172. Deflection direction 178 typically corresponds to the direction of propagation of the acoustic wave in the acousto-optic modulator. As acousto-optic modulator 120 turns on input beam 135, a cross-section 174 of the beam expands in direction 178. Accordingly, the initially illuminated part of region 170 is narrow and toward one edge until the input beam has a fully illuminated cross-section such as cross-section 175. Similarly, when acousto-optic modulator 120 turns off input beam 135, one edge of the input beam darkens first, and a shrinking cross-section 176 of the beam causes illuminated region 170 to recede toward the opposite edge. This reduces sharpness at the edges of illuminated regions formed by multiple scan lines, skews rectangular illuminated areas, and causes pattern lines at 45xc2x0 to the scan direction to differ in thickness from pattern lines at 135xc2x0 to the scan direction. However, to provide independent control of the beam intensities and a narrow scan brush, acoustic waves in an acousto-optic modulator generally propagate at an angle relative to the scan direction.
As shown in FIG. 1C, a separation 133 between beams 132, 134, 136, and 138 inside acousto-optic modulator 120 must be sufficient for acoustic waves 122, 124, 126, and 128 to independently modulate respective beams 132, 134, 136, and 138. Typically, separation 133 must be more than a beam diameter. To avoid the separation causing gaps between scan lines, a scanning direction 172 is selected so that beams 132, 134, 136, and 138 overlap when viewed along the scan direction 172. An advantage of overlapping beams is the narrow width 180 of the scan brush. Narrow brushes reduce scan line bow which is common for conventional f-xcex8 scan lenses. (Scan line bow is the curvature of scan lines that are off the optical axis of a scan lens.) Also, scanning overlapping beams along scan direction 172 forms a band of scan lines without intervening gaps, which simplifies indexing of scan lines to cover the image area. As indicated above, disadvantages of the configuration of FIG. 1C are reduced sharpness at the edges in the image, skew of rectangular areas, and 45xc2x0/135xc2x0 line thickness bias.
As shown in FIG. 1D, the scan direction 172 can alternatively be the same as or opposite to the direction of propagation of acoustic waves 122, 124, 126, and 128 in acousto-optic modulator 120. With this configuration, the separation 133 required for independent modulation of beams controls the separation between the scan lines. This creates a scan brush that is wider than the brush of FIG. 1C, and the wider scan brush increases scan line bow from a conventional f-xcex8 scan lens, making the accuracy required for integrated circuit applications difficult to achieve. Other types of scan lenses can reduce scan line bow but generally cause scan beams to move with non-uniform velocity and therefore can distort the image.
Systems and methods are sought that use simultaneous scan beams for faster scanning but avoid scan line bow and image distortion and also avoid the skew, blurred edges, and directional bias associated with acousto-optic modulators having acoustic waves propagating at an angle to the scan direction.
In accordance with the invention, a multi-beam scanner has a wide scan brush, a modulator that controls intensity of pixels in scan beams, an optical system that minimizes scan line bow at the expense of non-uniform scanning beam velocity, and a timing generator that generates a pixel clock signal having a variable period that compensates for the non-uniformity of pixel velocity. The wide separation of scan beams permits the modulator to turn beams on or off with a direction of brightening or darkening in the cross-section of the beams being opposite to the scanning direction. This allows the brightening direction to be opposite the scan direction to improve edge sharpness, avoid skew in rectangular regions, and avoid a directional bias in line thickness.
A novel arrangement of the beams in the brush permits a uniform indexing step size to uniformly expose an image region. In particular, a brush with b beams spaced a distance n apart uniformly covers the image region after repeated scanning and indexing by a distance m if the number of beams b and the distances n and m are such that the ratio of m to n is equal to the ratio of b to an integer q that has no common factors with b. In one embodiment, a diastemal brush has a top half including b beams uniformly spaced distance n apart and a bottom half including b beams uniformly spaced distance n apart. The distance between the top and bottom halves is 1.5*n. With this diastemal brush and a uniform indexing distance m, the top half forms uniformly spaced scan lines, and the bottom half forms scan lines midway between adjacent scan lines that the top half forms. Other embodiments of the scan bush include three or more sections of equal spaced beams separated by two or more diastema.
In one embodiment, the timing generator includes: a source of pixel period values and a counter. The counter loads a first part of a pixel period value selected for a pixel, counts for a period of time indicated by the first part, and asserts a signal marking an end of the period. An additional delay calculator circuit can delay the signal from the counter for a time shorter than the period of a clock signal to the counter. A second part of the pixel period value controls the delay. The combination of the times for the count and the delay forms the complete pixel period. After asserting a pulse for the pixel clock for a pixel period, the source supplies the next pixel period value which controls the count and delay for the next pixel period.
In alternative embodiments, the source of the pixel period values includes a set of registers, a register and a series of adders, or a look-up table. In one embodiment, the source of pixel period values includes a lookup table, a start index register, and a pixel counter, which initially loads from the start index counter and supplies an address to a lookup table. When a set of registers or a register and a series of adders provide the pixel period values, a multiplexer selects the pixel period value according to a select signal from a look-up table. The look-up table is indexed by pixel and selects an appropriate pixel period value for each pixel. The pixel counter increments the pixel index each time the timing generator marks a boundary of a pixel, and in response to the changed pixel index, the timing generator selects the next pixel period value.